1. Field of the Invention
This invention relates generally to coating thickness measuring devices, and more particularly to an instrument and method for the non-destructive contactless measurement of contact sensitive, thin coatings on basal substrates by beta-ray backscatter techniques and, even more specifically, to an improved system for precisely locating the beta-ray source of a measuring instrument in a non-contacting position with respect to the specimen to be measured.
2. Description of the Prior Art
Beta-ray backscatter measuring instruments have been extensively utilized to measure the thickness of metallic deposits and coatings of various materials such as, for example, the conductive plating on printed circuit boards or the like. These instruments generally include a source of beta radiation, conveniently a radio-active isotope. This source emits radiation which is directed to strike a metallic coating and the radiation backscatter from the coating is measured by a detector in the form of a Geiger-Mueller tube. An associated electronic counter or readout unit converts the output of the detector into a usable form.
The accuracy and sensitivity of the beta-ray backscatter instrument is largely dependent upon the geometry of the system, that is, the geometric or positional relationship between the source, work piece and detector. To this end auxiliary means for locating the work piece relative to the source and detector are usually incorporated, in accordance with the dictates of the work piece configuration, as a component of most such measuring systems.
Although the beta-ray backscatter instrument is highly useful in taking measurements of thin coatings, problems have arisen in properly and precisely locating the source and detector with respect to the work piece.
Another problem with present beta-ray backscatter techniques is that, heretofore, a contact method has often been used. The sample is placed in contact with the metallic surface over which it is then scanned by the Geiger-Mueller detector. This contact is generally not a problem in the plating industry because the metallic surfaces are highly forgiving of contact with other metal surfaces. However, in certain cases, contact of any type is detrimental when the surface to be measured is very soft or very brittle. For example, thickness measurements requiring direct physical contact with epitaxial coatings of mercury cadmium telluride (HgCdTe) or of cadmium telluride (CdTe) electro optical films result in damage to or destruction of the coating.
Several methods have been used and are being used to determine the thickness of epitaxial deposits. In the case of HgCdTe or CdTe, one method has been to use destructive cleave measurements. In this method, the deposited substrate is broken up by cleaving it along a fracture line. A cross-section of that piece is then microscopically examined to visually determine the thickness of the epitaxial coating. This method has the obvious disadvantage of being destructive and, therefore, is not suitable for a production line type determination.
Non-destructive method of determining the thickness of epitaxially deposited coatings is to measure optical interference fringes. This is accomplished with the use of an infrared spectrometer by performing an optical transmission in the desired infrared region. Results from such a transmission are sine-shaped interference fringe patterns. The period of these fringes, in other words, the distance between peaks of the sine waves plotted by the infrared spectrometer, is calculatable back to what the thickness of the epitaxial deposit is. This particular method is well known in the art, compatible with production line environments, and is commonly used. (For example, see an article entitled "Transmittance and Reflectance of a Coated Substrate with Application to Index Measurement of Thin Films", by J. SooHoo and R. D. Henry, J. Appl. Phys., Vol. 49, No. 2, February 1978.) One disadvantage of this method, however, is that the measuring instrument itself is very expensive. The process is time consuming, taking several minutes for each determination. Further, the sample to be measured must be placed in a chamber and purged of air so there is additional waiting time incurred before a measurement can even be taken. A measurement using the infrared spectrometer typically runs approximately three minutes followed by an additional three minute purge process. In addition, a standardization procedure must be performed periodically for calibration. Another major disadvantage to the use of the infrared spectrometer is that not only is it time consuming but also that it is not adequate for use with samples where the coating or the substrate has uneven or irregular surfaces. An irregular surface results in an interference pattern which is not readily interpretable.
Another approach to thickness determination which is destructive to the sample, is scanning electron microscope (SEM), auger, or similar instrumental analysis. These analyses require samples to be sectioned into small pieces and then subjected to time consuming and destructive measurement. This particular method is acceptable for production spot analysis but is certainly unsuitable for in-line production thickness measurements. An article of interest in this area appeared in Test and Measurement World, pgs. 76-82, September 1983, entitled "Analyzing Semiconductors with Auger Spectroscopy."
A method and apparatus for the contactless determination of the thickness of a coating on a work piece is described in U.S. Pat. No. 4,771,173 to Weismuller. The device there described includes a beta-ray backscatter instrument for measuring the coating thickness. The beta-ray backscatter measuring instrument includes a radiation source, and a radiation detector to measure the amount of backscatter, which is correlated to the thickness of the coating being measured. The radiation source and the radiation detector are carried in a measuring objective, which includes an aperture that is brought into an close but non-contacting position with respect to the area of the coating for which the thickness is to be measured. The distance between the measuring objective and the work piece is accurately and repeatedly controlled by the use of an optical focussing system. Such a system provides highly repeatable results, and is extremely useful in many applications. Nevertheless, because of the optical focussing requirements, that system is not readily adaptable to automation. For use in production environments, automation is desirable both for increased production throughput, and to permit more measurements to be taken on each wafer (work piece) for more accurate contour mapping.